Modern motor vehicle air brake systems, and particularly such systems as are now used on large trucks, provide greater braking capacity than systems provided in the past. Although this increased capacity provides better braking characteristics for heavily loaded vehicles, this increased capacity also increases the sensitivity of the brakes during stopping of a lightly loaded vehicle. This makes it more difficult for the operator to execute a smooth stop, particularly at low speeds, with a lightly loaded vehicle and makes the front wheel brakes more sensitive at low system pressures.
Brake systems which include air brake blend back proportioning valves to overcome this problem (that is, valves which provided an outlet pressure proportionally reduced from the inlet pressure at lower braking pressures and which blend back or equalize inlet and outlet pressures at higher braking pressures) are shown in U.S. Pat. Nos. 3,453,029 and 3,769,997 and in pending U.S. patent application Ser. No. 475,059, filed May 31, 1974.
The present invention departs from prior art air brake systems by providing an air brake system in which the proportioning valve includes a shuttle piston, a blend back sleeve, and a one-piece outlet diaphragm. The outlet diaphragm seals a reference chamber from outlet pressure and permits movement of the shuttle piston and blend back sleeve together as well as movement of the shuttle piston and blend back sleeve relative to one another. The proportioning valve also includes a one-piece inlet diaphragm which seals the reference chamber from inlet pressure and which provides a valving surface to open and close communication between the inlet and the outlet of the valve.